1. Field of the Invention
The invention relates to a CCD (charge-coupled device) image sensor, and more particularly to a CCD image sensor in which signal charges received from a plurality of charge transfer devices are input into a charge-detecting capacitor to which the charge transfer devices are connected.
2. Description of the Related Art
A CCD image sensor including a plurality of charge transfer devices is recently required to include photodiodes fabricated in a smaller size and have a higher resolution. In order to fabricate photodiodes in a smaller size, it is necessary to fabricate a charge transfer device in a smaller size. However, it would take much time and cost for doing so, and hence, it is quite difficult to accomplish fabrication of a charge transfer device in a smaller size in response to requirement of fabrication of a photodiode in a smaller size. Hence, a photodiode has been conventionally fabricated small and the number of photodiodes has been conventionally increased without fabrication of a charge transfer device in a smaller size, as follows.
FIG. 1 is a view of a conventional CCD image sensor.
The illustrated CCD image sensor 200A is of a single CCD type. Specifically, the CCD image sensor 200A includes a photodiode row 202 comprised of a plurality of photodiodes arranged in a line, and charge transfer devices 201 arranged in a line in facing relation with the photodiode row 202. Each of the photodiodes in the photodiode row 202 outputs signal charges to the charge transfer device 201 through a reading gate 210.
The photodiodes in the photodiode row 202 are arranged at a 8 micrometer pitch, for instance, and the charge transfer devices 201 are fabricated in accordance with a pitch of the photodiode row 202. A signal is applied to a charge-transfer electrode (not illustrated) in each of the charge transfer devices 201, and resultingly, charges having been ejected from the photodiodes are transferred to an output gate 203 through the charge transfer devices 201. Charges pass through the output gate 203, a charge-detecting capacitor 206 and a source follower circuit 207, and then, are output from the CCD image sensor 200A as output signals.
FIG. 2 is a view of another conventional CCD image sensor which is of a dual CCD type, suggested in Japanese Patent Application Publications Nos. 11-164087 and 1-248665.
The illustrated CCD image sensor 200B includes charge transfer devices 201a and 201b outside a photodiode row 202. The photodiode row 202 includes a first group of photodiodes which eject charges to the charge transfer devices 201a and a second group of photodiodes which eject charges to the charge transfer devices 201b. The photodiodes in the first group and the photodiodes in the second group are alternately arranged at a certain pitch, for instance, at a 4 micrometer pitch. The charge transfer devices 201a or 201b may be arranged at a pitch equal to a pitch at which the charge transfer devices 201 illustrated in FIG. 1 are arranged.
The photodiode row 202 ejects charges to the charge transfer devices 201a and 201b through reading gates 210. The thus ejected charges are transferred through the charge transfer devices 201a and 201b, and alternately input into a common output gate 203.
The output gate 203, a charge-detecting capacitor 206 and a source follower circuit 207 may be arranged for each of the charge transfer devices 201a and 201b for independently outputting charges transferred through the charge transfer devices 201a and 201b. However, the CCD image sensor would be necessary to include a switch for allowing the charges transferred through the charge transfer devices 201a and 201b to be output.
The CCD image sensor 200B is designed to have the output gate 203 acting as a common output gate for the charge transfer devices 201a and 201b so as to omit such a switch as mentioned above.
The dual CCD type CCD image sensor 200B can have photodiodes in the number twice greater than the single CCD type CCD image sensor 200A by including charge transfer devices fabricated in accordance with a process identical with a process in accordance with which the charge transfer device 201 in the single CCD type CCD image sensor 200A is fabricated, and having the same length as that of the charge transfer device 201. That is, a dual CCD type CCD image sensor can have photodiodes in the doubled number relative to a single CCD type CCD image sensor without fabricating charge transfer devices in a small size. Fabrication of a photodiode in a small size is not so difficult in comparison with fabrication of a charge transfer device in a small size.
FIG. 3 is a view of another conventional CCD image sensor including two photodiodes arranged in staggered arrangement, suggested in Japanese Patent Application Publication No. 2001-203342.
The illustrated CCD image sensor 200C includes first and second charge transfer devices 201a and 201b in facing relation to first and second photodiode rows 202a and 202b. Photodiodes in the first diode row 202a and photodiodes in the second diode row 202b are staggered by a half pitch to each other.
The CCD image sensor 200C is designed to have two single CCD type CCD image sensors 200A (see FIG. 1) arranged such that photodiodes in two photodiode rows are staggered by a half pitch, and further have a common output gate 203 through which signals are output. Such a structure allows the CCD image sensor 200C to have photodiodes in the number twice greater than the single CCD type CCD image sensor 200A without necessity of fabricating a charge transfer device in a small size, similarly to the dual CCD type CCD image sensor 200B.
In the dual CCD type CCD image sensor 200B and the CCD image sensor 200C including photodiodes arranged in staggered arrangement, it is possible to set a frequency of a signal applied to a charge-transfer electrode in a charge transfer device, equal to a half of a frequency of the same in the single CCD type CCD image sensor 200A under condition that charges are ejected from photodiodes in the common number and in a common period of time. This ensures prevention of electromagnetic interference (EMI).
In addition, the CCD image sensor 200C has advantages relative to the dual CCD type CCD image sensor 200B that a photodiode can be fabricated in a larger size, ensuring a higher signal-to-noise (SIN) ratio and a broader dynamic range.
FIG. 4 is a view of another conventional CCD image sensor including four photodiodes arranged in staggered arrangement, and FIG. 5 is a view of still another conventional CCD image sensor including four photodiodes arranged in staggered arrangement.
Each of the CCD image sensor 200D illustrated in FIG. 4 and the CCD image sensor 200E illustrated in FIG. 5 includes four photodiode rows 202a to 202d wherein photodiodes in the photodiode rows 202a to 202d are staggered by a quarter pitch relative to one another, and presents photodiodes in the number twice greater than the CCD image sensors illustrated in FIGS. 2 and 3.
The CCD image sensor 200D illustrated in FIG. 4 includes two rows of charge transfer devices 201a and 201b. The first and second photodiode rows 202a and 202b are commonly connected to the charge transfer device 201a, and the third and fourth photodiode rows 202c and 202d are commonly connected to the charge transfer device 201b. 
For instance, when one of the photodiode rows 202a and 202b both connected to the charge transfer devices 201a ejects charges into the charge transfer device 201a, in other words, when one of the photodiode rows 202a and 202b uses the charge transfer device 201a, charges ejected from the other of the photodiode rows 202a and 202b are exhausted through a charge-drainer 223a or 223b. The same is applied to the third and fourth photodiode rows 202c and 202d. Thus, if the first and fourth photodiode rows 202a and 202d use the charge transfer devices 201a and 201b, charges ejected from the second and third photodiode rows 202b and 202c are exhausted through a charge-drainer 223b. 
In the CCD image sensor 200D, charges ejected from the photodiode rows 202a to 202d are separately output twice.
In the CCD image sensor 200E illustrated in FIG. 5, output signals transmitted from two dual CCD type CCD image sensors are switched by a switch 214. In the CCD image sensor 200E, charges ejected from one of the first and second photodiode rows 202a and 202b and charges ejected from one of the third and fourth photodiode rows 202c and 202d are input at the same timing into associated charge-detecting capacitors 206a and 206b. Hence, the CCD image sensor 200E is necessary to include a switch such as the switch 214. However, since it is not necessary separately output twice charges ejected from the photodiode rows 202a to 202d, it would be possible to shorten a period of time necessary for outputting signals, in comparison with the CCD image sensor 200D illustrated in FIG. 4.
As mentioned above, a CCD image sensor including four photodiodes arranged in staggered arrangement is advantageous for increasing the number of photodiodes, but is accompanied with problems that charges have to be ejected from photodiodes twice or half by half, and that signals have to be switched by means of the switch 214. These are because charges transferred through the four rows of charge transfer devices cannot be input into a common charge-detecting capacitor.
Japanese Patent Application Publication No. 10-233883 has suggested a CCD image sensor designed to output charges having been transferred through each of three or more charge transfer devices, through a common charge-detecting capacitor. In the suggested CCD image sensor, charges ejected from three photodiode rows associated with red (R), green (G) and blue (B) are output through a common charge-detecting capacitor. Signals for each of the colors are amplified in a common amplifier to thereby reduce linearity error in color images.
FIG. 6 is a block diagram of a color CCD image sensor suggested in the above-mentioned Publication.
The illustrated color CCD image sensor 300 includes photodiodes 312R, 312G and 312B for RGB colors, CCD shift registers 310R, 310G and 310B for RGB colors, and output gates 313R, 313G and 313B for RGB colors.
Charges ejected from the photodiodes 312R, 312G and 312B are transferred to the output gates 313R, 313G and 313B through the CCD shift registers 310R, 310G and 310B.
Two phase driven signals φ1 and φ2 are applied commonly to the shift registers 310R, 310G and 310B, and gate control signals Rog, Gog and Bog are applied to the output gates 313R, 313G and 313B, respectively. Charges transferred through any one of the output gates 313R, 313G and 313B are input into a floating source 314 as a charge-detecting capacitor which is common to red, green and blue. Charges having been input into the floating source 314 are output to an amplifier (not illustrated) through a source follower circuit 318. Thus, signals for each of the colors can be amplified by means of a common amplifier without using a switch, ensuring reduction in linearity error in color images.
However, the CCD image sensor suggested in the above-mentioned Publication is accompanied with a problem that photodiodes cannot be arranged in a high density, because arrangement of photodiodes is identical with the photodiode arrangement in a single CCD type CCD image sensor illustrated in FIG. 1. In addition, it is unavoidable that a channel length from each of the shift registers 310R, 310G and 310B to the floating source 314 is lengthy. Hence, when charges having been transferred through the shift registers 310R, 310G and 310B are input into the floating source 314, charges can hardly be transferred an area located just below the output gates 313R, 313G and 313B.
In general, charges transferred through a plurality of charge transfer devices are output through a common charge-detecting capacitor, a P+ diffusion layer is formed extending to an area just below an output gate in order to prevent charges transferred through a plurality of charge transfer devices from being mixed with one another. That is, charge transfer devices are separated from one another even at an area just below output gates. However, this is accompanied with a problem that since an area at which charges join with one another becomes narrower in width at a location closer to a charge-detecting capacitor, a path through which charges are transferred is made narrow due to a P+ diffusion layer, and hence, a P+ diffusion layer is close to an adjacent P+ diffusion layer, resulting in narrow-channel effect. If narrow-channel effect is caused, a potential is lowered, and hence, mobility speed of charges is reduced.
Japanese Patent Application Publication No. 11-205532 has suggested a solid-state image sensor including first, second and third photodiode rows. Shift electrodes and CCD registers are located between the first and second photodiode rows and further between the second and third photodiode rows. Outside the first and third photodiode rows are arranged shift electrodes and CCD registers.
Japanese Patent Application Publication No. 64-14966 has suggested a charge transfer device including a charge-transfer electrode and an output gate electrode both formed in a semiconductor substrate having a first conductivity. A charge-detecting region having a second conductivity is formed in the semiconductor substrate just below the output gate electrode. A charge-transfer channel located just below the charge-transfer electrode is narrowed towards the charge-detecting region. A stepped potential is formed below the charge-transfer electrode.
Japanese Patent Application Publication No. 4-14842 has suggested a charge-detecting circuit in a charge transfer device, including two rows of charge transfer registers, a floating diffusion type charge-reader which alternately reads out signal charges from final stages of the charge transfer registers, and an output gate located between the final stages of the charge transfer registers and the floating diffusion type charge-reader. Charges alternately read out of the final stages of the charge transfer registers are input into the floating diffusion type charge-reader through a single transfer channel formed below the output gate.